Conventional microelectronics, which are based on silicon components such as CMOS chips (complementary metal oxide semiconductor chips) are reaching their limits, even with the further progress in miniaturization. Molecular electronics are being discussed as one of the possible ways for further component miniaturization.
In addition to the general problem of developing circuit elements with the aid of molecular electronics, a further aspect that is considered in this context is the development of alternatives to the previous semiconductor memory elements such as DRAMs (dynamic random access memories), SRAMs (static random access memories) or flash memories.
It is known from C. P. Collier et al., Electronically configurable molecular-based logic gates, Science, Vol. 285, pages 391-394, 1999, that configurable circuit elements which can be used to construct logic gates can be obtained with the aid of monomolecular layers based on rotaxanes. This is based on the fact that the monomolecular layers of the rotaxanes can be changed, that is to say switched, from a conductive state to a less conductive state by application of a voltage. However, this switching process that is known from C. P. Collier et al. is irreversible, and is thus suitable only for a write once/read multiple application.
It is known from C. P. Collier et al., A Catenane-based solid state electronically reconfigurable switch, Science, Vol. 289, pages 1172-1175, 2000, that a reversible switching process can be achieved with the aid of a further specific molecule class, the so-called catenanes. However, considerably weaker signals are observed with this switching process.
The two circuit elements that are known from C. P. Collier et al., Electronically configurable molecular-based logic gates, Science, Vol. 285, pages 391-394, 1999, and C. P. Collier et al., A Catenane-based solid state electronically reconfigurable switch, Science, Vol. 289, pages 1172-1175, 2000, have even further disadvantages for widespread practical application, however. On the one hand rotaxanes and catenanes can be obtained only by complex synthesis processes. On the other hand, Langmuir-Blodgett methods are used to produce the monomolecular layers for both circuit elements. Furthermore, the suitability of these Langmuir-Blodgett methods for the coating of surfaces of components such as silicon wafers, which are normally used to produce electrical components, is still uncertain.
In addition to the approaches discussed above for development of circuit elements based on organic molecule layers, it is known from D. I. Gittins et al., A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups, Nature, Vol. 408, pages 67-69, 2000, that an electrical switch can be produced by the specific combination of a molecule with a bispyridinium unit and a nanoparticle (metal cluster) composed of gold. Since nanotechnology is still in its infancy, it is questionable whether this system can still be used for a practicable application for the foreseeable future.
Further developments in the field of molecular electronics are described in M. A. Reed et al., Prospects for molecular-scale devices, IEEE, Tech. Digest, pages 227-230, 1999. Inter alia M. A. Reed et al. describes a circuit element based on a specific substituted 4,4′-di(phenylene-ethynylene)-benzothiolate, which bears an ethyl group as a substituent at the 3-position in the phenylene unit. A temperature-dependent conformational change, specifically a rotation of the central phenylene unit, in this circuit element results in an abrupt change in the conductivity at 20 K. Another phenylene-ethynylene-benzothiolate which is substituted at the 4-position in the central phenylene unit with a nitroamine group which is used as the redox center, can be used in a memory element, according to M. A. Reed et al.
However, practical use of these circuit elements is described in M. A. Reed et al. as not being certain. The benzothiolates discussed in M. A. Reed et al. are referred to in C. Joachim et al., Electronics using hybrid-molecular and mono-molecular devices, Nature, 408, pages 541-548, 2000, only as being promising for memory elements.
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